Magnetic fluids or ferrofluids, these two terms being used interchangeably in this application, are well known in the art. Ferrofluids are described in "Magnetic Fluids," International Science and Technology, July 1966, p. 48-56; U.S. Pat. No. 3,215,572; in the publication of R. Kaiser and G. Miskolczy, "Magnetic Properties of Stable Dispersions of Subdomain Magnetite Particles," J. Applied Physics, Vol. 41, No. 3, Mar. 1, 1970, pp. 1064-1072; in "A Catalog of Magnetic Fluids", Ferrofluidics Corporation, Burlington, Mass., 1972 and elsewhere.
Magnetic fluids or ferrofluids are used as sealants to provide a positive hermetic barrier against gas and other contaminants independent of differential pressure or direction. They have enjoyed particular utility as seals in many rotary shaft systems and exhibit long life, high reliability, low or immeasurable discharge and cost effectiveness. In principle, ferrofluid seals operate through the use of a ferrofluid in a gap between rotary and stationary surfaces in the presence of a magnet to provide focused radial magnetic flux lines which retain the ferrofluid in the gap about a rotating shaft.
The use of ferrofluids in shaft systems involving linear motion of the shaft has not proven to be as satisfactory as in rotary shaft systems. The difficulty in a shaft system involving linear motion of the shaft is the inherent depletion of the ferrofluid as a result of the wiping action of the shaft during linear motion. One approach to the solution of this problem has been the development of certain linear drive shaft seals as exemplified by U.S. Pat. Nos. 4,284,605 and 4,309,040.
A linear-motion valve is a particular example of a shaft system employing linear motion. Such a valve, sometimes called a push-pull valve, generally consists of a cylindrical valve spool, slideably disposed through a circular seal assembly. The valve spool contains an axial bore-like hole extending from its outer end to a point near its inner end and one or more radial holes connecting the inner end of the bore-like hole to the periphery of the valve spool. In a typical application, the circular seal assembly would be disposed in the wall of a vessel. Suitable stops are usually provided on the valve spool to permit or facilitate positioning the valve spool in either its open or closed position. Under high differential pressure conditions the detent holding action of the magnet on the magnetically permeable rings would be exceeded thereby necessitating a simple spring ball detent action on radial grooves in the spool to prevent the spool from moving from the set position.
It is an object of this invention to provide an improved valve assembly employing a linear motion valve spool and a magnetic fluid as the sealant wherein the loss of sealant due to the wiping action of the valve spool during its linear motion is minimized. Other objects will become apparent from the description of the invention.